Amateur astronomers’ ground-based observations of spoke activity before Saturn’s 2025 equinox

Introduction

Weak elongated features called spokes are observed by professional instrumentation in the B-Ring during Saturn’s equinoxes in 1980 by Voyagers [1], 1995 by Hubble Space Telescope [2] and 2009 by Cassini [3].

Amateur observations

On the other hand, amateur astronomers with their ground-based telescopes could also observe spokes for the first time during 2009 equinox [4] despite a lesser resolution on their images.

Spoke characteristics

Previous equinoxes professional studies could show that spoke activity:

• is observable on both sides of the rings (but mostly on morning ansa, part of the ring emerging from Saturn’s shadow [6]), in B-ring.
• has a period corresponding to the period of SKR (Saturnian Kilometric Radiation) emission [7]
• mainly occurs around 200° SLS4 longitude
• is mainly visible at low sun declination (between -10° and +10°)

Current equinox amateur observations

After the end of Cassini mission in 2017, the study of spoke activity can now only rely on space telescopes’ or amateur astronomers’ observations for the current 2025 equinox [5].

Before 2025 Saturn rings-plane crossing, 39 possible spoke amateur observations have been identified. The best observations are performed in red/infrared wavelengths, or CH₄ 889nm absorption band where the dark spokes contrasts best with the bright B-ring. Animations could show spokes rotating with the planet.

The properties of those observations matches in general the previous characteristics. This shows that amateur astronomers can provide key data for continuing monitoring spoke activity.

 

References
[1] D'aversa E. et al 2013 - Saturn ring spokes an overview of their near-infrared spectral

[2] McGhee C. et al. 2005 - HST observations of spokes in Saturn's B ring - Icarus 173

[3] Callos S. R. et al. 2025 - A Survey of Cassini Images of Spokes in Saturn’s Rings Unusual - Planetary Science Journal Jan. 2025

[4] Delcroix M. et al.- First ground observations of Saturn's spokes around 2009 equinox-EPSC-DPS2011-336

[5] Simon A. et al. 2023 - Hubble Detects the Start of a New Saturn Ring Spoke Season - Geophysical Research Letters

[6] Hirata N.  et al. 2022 - Dust release from cold ring particles as a mechanism of spoke formation in Saturn’s rings - Icarus 2022

[7] C.J. Mitchell et al. 2013 - The behavior of spokes in Saturn’s B ring - Icarus vol. 225 Issue 1 July 2013